This invention relates to low-power transformers for use in cell phone chargers or the like, and more particularly to a planar transformer suitable for installation on a printed circuit board in miniaturized electronic equipment.
Power supplies for miniaturized electronic equipment such as cell phone chargers typically involve converter circuits, such as, for example, flyback converters, that require a very low-power (2-3 watts) transformer for their operation. Because of the space constraints in such equipment, the transformer is typically a limiting factor in the power supply design. Inasmuch as the power supply usually takes the form of a low-profile printed circuit board, it is desirable to make the transformer as small and flat, or planar, as possible, and to so arrange its pins that it can either be mounted in a through-hole configuration or soldered directly to pads on the board.
The present invention is an improvement over my copending application Ser. No. 09/545,367 filed Apr. 7, 2000 and assigned to the assignee of this application. In that application, I disclosed a transformer construction in which the primary and secondary winding bobbin structures are positioned one above the other in the axial direction (i.e., a direction perpendicular to the plane of the transformer on each side of an electrostatic shield). The construction of the ""367 application is relatively flat when mounted to an edge of a circuit board, but loses that advantage when mounted in a through-hole configuration by pins. It is therefore desirable to provide a transformer that can be mounted on a circuit board by pins yet lies as flatly against the board as possible.
The present invention fills the above-stated need by providing a transformer construction in which the secondary bobbin structure fits wholly inside the primary bobbin structure, yet is electrostatically shielded therefrom and so configured as to minimize creepage and leakage inductance between the primary and secondary windings. The primary bobbin structure and the secondary bobbin structure are also so configured as to easily snap together during assembly of the transformer and are light and sturdy enough to be impact-resistant if the power supply or the equipment using it is dropped.
Broadly stated, the present invention comprises a planar transformer for printed circuit boards, comprising: a primary bobbin structure; a secondary bobbin structure; and a core extendable through a central opening in each of said bobbin structures; wherein the primary bobbin structure includes a body surrounding said opening, a recess formed in said body around said opening, a primary bobbin supported by said body, and primary and secondary connection pins for mounting said primary bobbin structure on a circuit board; and wherein the secondary bobbin structure forms a secondary bobbin that is lockingly insertable into said recess so as to be contained therein.